Biomass has been a primary source of energy over much of human history. During the late 1800's and 1900's the proportion of the world's energy sourced from biomass dropped, as the commercial development and utilization of fossil fuels occurred, and markets for coal and petroleum products dominated. Nevertheless, some 15% of the world's energy continues to be sourced from biomass, and in developing countries the contribution of biomass is much higher at 38%. In addition, there has been a new awareness of the impact of the utilization of fossil fuels on the environment. In particular, the contribution of greenhouse gases, as a result of consuming fossil fuels.
Biomass, such as wood, wood residues, and agricultural residues, can be converted to useful products, e.g., fuels or chemicals, by thermal or catalytic conversion. An example of thermal conversion is pyrolysis where the biomass is converted to a liquid and char, along with a gaseous co-product by the action of heat in essentially the absence of oxygen.
In a generic sense, pyrolysis is the conversion of biomass to a liquid and/or char by the action of heat, typically without involving any significant level of direct combustion of the biomass feedstock in the primary conversion unit.
Historically, pyrolysis was a relatively slow process where the resulting liquid product was a viscous tar and “pyroligneous” liquor. Conventional slow pyrolysis has typically taken place at temperatures below 400° C., and over long processing times ranging from several seconds to minutes or even hours with the primary intent to produce mainly charcoal and producing liquids and gases as by-products.
A more modern form of pyrolysis, or rapid thermal conversion, was discovered in the late 1970's when researchers noted that an extremely high yield of alight, pourable liquid was possible from biomass. In fact, liquid yields approaching 80% of the weight of the input of a woody biomass material were possible if conversion was allowed to take place over a very short time period, typically less than 5 seconds.
The homogeneous liquid product from this rapid pyrolysis, which has the appearance of a light to medium petroleum fuel oil, is a renewable fuel oil. In particular, the renewable fuel oil may be an unenriched renewable fuel oil, formed from a biomass comprising cellulosic material, wherein the only processing of the biomass may be a therma-mechanical process (specifically comprising grinding and rapid thermal processing, with no post or further catalytic processing, hydrogenation, or enrichment or other chemical upgrading of the liquid prior to its use as a combustion or renewable fuel).
In practice, the short residence time pyrolysis of biomass causes the major part of its organic material to be instantaneously transformed into a vapor phase. This vapor phase contains both non-condensable gases (including methane, hydrogen, carbon monoxide, carbon dioxide and olefins) and condensable vapors. It is the condensable vapors that constitute the final liquid product, when condensed and recovered, and the yield and value of this liquid is a strong function of the method and efficiency of the downstream capture and recovery system.
Given the fact that there is a limited availability of hydrocarbon crude and an ever increasing demand for energy, particularly liquid transportation fuels, alternative sources are therefore required. The abundance and sustainability of biomass makes renewable feedstock an attractive option to supplement the future demand for petroleum. The difficulty with biomass is the fact that it contains oxygen, unlike conventional hydrocarbon fuels, and historically has not been readily convertible into a form that can be easily integrated into existing hydrocarbon based infrastructure. In particular, utilization of unenriched pyrolysis oil as a heating oil or fossil fuel substitute has been limited due to its lower energy density, lower combustion temperature, relative thermal instability, corrosiveness, and limited miscibility with traditional heating oil or fossil fuels.
The lower energy density, lower combustion temperature, and poor thermal stability of unenriched pyrolysis oil are attributable in part to high water content (typically >20 wt. %) and the presence of oxygenated hydrocarbons (typically >40 wt. %). The oxygenated compounds, including carboxylic acids, phenols, cresols, and aldehydes, tend to undergo secondary reactions during storage, resulting in increased viscosity, phase separation and/or solids formation. Additionally, pyrolysis oil contains char and alkali metal contaminants which appear to catalyze these secondary reactions, further contributing to the stability problems.
As a result of the stability problems, storage of pyrolysis oil for use as a heating oil or fossil fuel substitute in combustion systems can be problematic. In particular, viscosity changes can occur at ambient storage temperature and may accelerate at higher temperatures. Moreover, rapid temperature changes can lead to phase separation of the pyrolysis oil into an aqueous-rich phase and an aqueous deficient phase. These changes may render the pyrolysis oil unsuitable for handling in conventional or existing fossil fuel-based infrastructure and equipment, including pumps, vessels, and boiler systems.
The corrosiveness and limited miscibility of pyrolysis oil are due largely to its acidity and its high moisture and oxygen contents. Pyrolysis oil typically has a pH<3 and a TAN>150, making it corrosive to storage, pipes, existing fossil fuel-based infrastructure and equipment, including pumps, vessels, and boiler systems. In addition, the presence of char and alkali metals contribute to ash formation during combustion of pyrolysis oil. As a result, unenriched pyrolysis oil is not immediately compatible with existing liquid and/or fossil fuel-based infrastructure as a heating oil or fossil fuel substitute.
Upgrading pyrolysis oil to overcome the foregoing difficulties has proven to be a difficult challenge. The use of catalytic cracking of a solid or liquid biomass, a biomass-derived vapor, or a thermally-produced liquid as a means to produce hydrocarbons from oxygenated biomass is technically complex, relatively inefficient, and produces significant amounts of low value byproducts. To solve the catalyst and yield issues, researchers looked at stand-alone upgrading pathways where biomass-derived liquids could be converted to liquid hydrocarbons using hydrogen addition and catalyst systems in conversion systems that were tailored specifically for the processing of oxygenated materials (Elliott, Historical Developments in Hydroprocessing Bio-oils, Energy & Fuels 2007, 21, 1792-1815). Although technically feasible, the large economies-of-scale and the technical complexities and costs associated with high-pressure multi-stage hydrogen addition (required for complete conversion to liquid hydrocarbon fuels) are severely limiting and generally viewed as unacceptable. Other approaches such as liquid-liquid extraction, or gasification face similar hurdles, significantly reducing the economic competitiveness of pyrolysis oil as a petroleum substitute.
New approaches are needed to circumvent the foregoing limitations. One innovative embodiment that forms part of the present application is a method of maintaining and handling an unenriched renewable fuel oil for use as a heating oil or fossil fuel substitute in a thermal system. Applicable thermal systems include a boiler, a furnace, a kiln, and an evaporative cooling system.